This invention relates to a method of and apparatus for plasma-MIG arc welding. In plasma-MIG (Metal-Inert-Gas) arc welding, a plasma arc is maintained in a gas flow between a non-consumable electrode and a workpiece, a consumable electrode is guided into the plasma steam coaxially therewith, and a MIG arc is maintained between the consumable electrode and the workpiece.
The terminal, or end, portion of the consumable electrode and the MIG-arc are both surrounded by a plasma sheath, which is produced by the plasma arc. Basically the effect of the plasma arc is to provide additional heating of the workpiece compared with the MIG-arc alone, resulting in a very wide range of different weld types and in extremely high deposition rates. In detail there are interactions between the plasma and MIG-arc systems which determine the field of use. The plasma arc and the MIG-arc have to have the same polarity, positive or negative, but need not be identical in voltage.
With both the electrodes positive a stable operating system is obtained, the position of the arc root on the consumable electrode being clearly defined within narrow limits. The MIG-arc takes whatever current is necessary to burn off the consumable electrode at the rate at which it is fed into the plasma stream. Therefore increasing the electrode feed rate in turn demands a greater current from the MIG power supply. The latter has a relatively flat output characteristic, such that the necessary current can be obtained without a major change in terminal voltage. It is possible to operate the MIG-arc over a very wide current range from low values up to several hundred amperes. The plasma arc current is however limited, by the overheating of the non-consumable electrode, to the order of 100A in the case of steel electrodes. For conditions wherein the MIG-arc takes less than about 250A, a deep penetration type weld is produced which has similar characteristics to a normal MIG-weld apart from the greater available heat input. However, for currents in excess of 250A the MIG arc gives a wide shallow deposit. This is due to the consumable electrode developing a helical spiral, so that the MIG-arc rotates in a circle of approximately the weld pool in size. This action depends in part on resistive heating of the consumable electrode to render it sufficiently soft.
With the electrodes negative, the non-consumable electrode is capable of carrying considerably higher current than with positive polarity without overheating, negative polarity being the normally-used polarity for plasma welding. Higher MIG-arc currents can be used without the wide shallow deposition effect. However, on negative polarity the MIG-arc is less stable in operation. Not only are the ranges of operating voltages and currents more restricted, but also care is needed in the balance between the plasma gas and the external shielding gas stream as well as careful adjustment of the stand-off distance between the nozzle and the workpiece. The cause of the instability found on negative polarity is due to the behaviour of the arc on the consumable electrode. In the inert atmosphere of the plasma gas, for instance pure argon, the cathode root tends to wander along the negative consumable electrode. The arc root is apparently subjected to excursion on the electrode and may cause disturbance to the plasma-MIG system. This may lead to either the arc climbing the consumable electrode and fusing the contact tube on the one hand, or at times the consumable electrode feeds down, touches the workpiece and causes the plasma arc to transfer to the consumable electrode and, hence extinguish the plasma arc.